Fourteen asthma patients will be enrolled in the study and each subject will visit the centre on five occasions (three pre-treatment visits and two post-treatment visits). We will evaluate pre-therapy optimization using inflammometry, pre-therapy ventilation defect reproducibility and BT treatment effects (pre-treatment visit 1= 5±2 days pre-optimization, pre-treatment visit 2 = 15±7 days pre-BT, pre-treatment visit 3 = 5±2 days pre-BT, post-treatment visit 4=6 weeks post-BT, post treatment visit 5 = 6 months post-BT). At pre-treatment visit 1, patients will be randomized to image-guided or conventional therapy. For all subjects, at all visits, spirometry, plethysmography and 3He and/or 129Xe MRI will be performed. Methacholine challenge (MCh) will be performed (for those subjects with FEV1 >60%) at pre-treatment visit 3 and post-treatment visit 4 with spirometry and MRI performed pre- and post-MCh. Pre-treatment visit 3 will include a low-dose thoracic CT that will be used to generate a detailed 3D model of the airway tree. MRI will be co-registered to the CT with 3D airway rendering to enable spatial comparisons between ventilation defects and airways.